In a conventional four-cycle internal combustion engine, the fuel and lubricating systems are maintained completely separate. Despite wide use, this division in the modern engine entails a number of shortcomings. For example, the oil is relied upon to not only reduce friction and wear, but also to serve as a coolant, an oxidation and corrosion inhibitor, and a transport fluid that removes wear metal particles and blow-by products (e.g., carbon, sludge, varnish, unburned fuel, and other combustion products) for subsequent filtration. Due to these requirements on the oil, the engine oil additives become depleted and the important characteristics of the lubricant are degraded. As a result, the oil over time will tend to experience an increase in viscosity and an accumulation of abrasive particles and oxides which, in turn, leads to the corrosion of engine components and increased wear. Moreover, replacement of the oil creates an added expense and a disposal problem with regard to the used oil. Finally, vehicles which are old or poorly maintained can experience considerable burning of the oil which leads to tailpipe emission problems.
A few engine systems have mixed oil and fuel together to facilitate oil replacement while the engine is in use. For instance, U.S. Pat. Nos. 5,431,138, 4,421,078, 4,869,346 and 4,495,909 disclose systems which pump a quantity of used oil into a fuel return line as the engine operates. Fresh oil in predetermined batches is also fed into the lubricating system to offset the oil which is removed. However, the maintenance of two fluid systems is still required. Moreover, as discussed above, the burning of oil creates undesirable pollution problems.
U.S. Pat. Nos. 4,572,120 and 4,615,305 to Matsumoto each discloses an outboard motor provided with a lubricant delivery tank mounted on the motor, and a storage tank which is mounted in the hull and fluidly coupled to the delivery tank. A pump feeds the lubricant in the delivery tank into the intake manifold of the motor. However, the outboard motor is a two-cycle engine, rather than a four-cycle engine. Moreover, this system requires the maintenance of separate oil and fuel systems and involves the burning of oil in the motor.
Other two-cycle, internal combustion engines have been produced which use an oil-fuel mixture for both lubrication and powering of the motor. However, these two-cycle engines are much different than modern four-cycle, internal combustion engines. For instance, these engines lack valves, rely upon oil-rich mixtures, and are very dirty engines which are not suitable for the high pollution standards now in existence for vehicles and other large engine applications.
Also, fuel lubrication is known to have advantages for an internal combustion engine, especially a diesel fuel engine. As a result, most diesel fuels have high lubricity, or contain lubrous additives, to ensure that the fuel injector pump and fuel injectors are adequately lubricated during normal operation. However, no four-cycle, internal combustion engine has been used in which the fuel serves as the lubricant for the engine.